An Frequency Division Duplex (FDD) mode and a Time Division Duplex (TDD) mode are widely used in radio communication systems. The FDD mode refers to a mode in which uplink and downlink use different frequency resources for communication. The TDD mode refers to a mode in which the uplink and downlink share the same frequency resources and the uplink communication and downlink communication are respectively performed through time domain division.
For example, a Long Term Evolution (LTE) system corresponding to an Evolved Universal Terrestrial Radio Access (E-UTRA) protocol developed by the 3rd Generation Partnership Project (3GPP) supports the FDD mode and the TDD mode, and a radio frame structure includes an FDD frame structure and a TDD frame structure.
FIG. 1 is a diagram illustrating the frame structure in the FDD mode. A 10 ms uplink radio frame and a 10 ms downlink radio frame are respectively composed of twenty 0.5 ms time slots which are numbered from 0 to 19. Time slots 2i and 2i+1 construct a subframe i of which the length is 1 ms. The uplink and the downlink are communicated at different frequency resources. FIG. 2 is a diagram illustrating the frame structure in the TDD mode. A 10 ms radio frame is composed of two half frames and the length of each half frame is 5 ms. One half frame includes five 1 ms subframes. A subframe i includes two time slots 2i and 2i+1 and the length of each time slot is 0.5 ms. The uplink and the downlink share the same frequency resources and the uplink communication or the downlink communication are performed at different subframes of the radio frame.
In the above two frame structures, for a Normal Cyclic Prefix (Normal CP), a time slot includes seven symbols and the length of each of the seven symbols is 66.7 us. In this case, the length of the CP of the first symbol is 5.21 us. The length of the CP of each of the remaining six symbols is 4.69 us. For an Extended Cyclic Prefix (Extended CP), a time slot includes six symbols and the length of each of the six symbols is 16.67 us.
Uplink/downlink configurations supported by the TDD mode are shown in Table 1. For each subframe in a radio frame, “D” represents a subframe dedicated to downlink transmission, “U” represents a subframe dedicated to uplink transmission, and “S” represents a special subframe of three fields including Downlink Pilot Time Slot (DwPTS), Guard Period (GP), and Uplink Pilot Time Slot (UpPTS). The length of each of DwPTS and UpPTS is shown in Table 2. In this case, the total length of DwPTS, GP, and UpPTS is 30720 T=1 ms, in which Ts is a time unit and is defined as 1/(15000*2048) sec. Each subframe i is represented by two time slots 2i and 2i+1 and the length of each time slot is Tslot, =15360·Ts, =0.5 ms.
The LTE TDD supports a 5 ms downlink-to-uplink switch-point periodicity and a 10 ms downlink-to-uplink switch-point periodicity. If the switch-point periodicity from the downlink to the uplink is 5 ms, the special subframe may exist in two half frames. If the switch-point periodicity from the downlink to the uplink is 10 ms, the special subframe only exists in the first half frame. Subframe 0, subframe 5, and DwPTS are always used for downlink transmission. UpPTS and a subframe following the special subframe are dedicated to uplink transmission. The configurations as shown in Table 1 may flexibly support different asymmetric services. The special subframe configurations as shown in Table 2 support GP with different lengths and different cell radiuses and avoid strong interference between base stations in a TDD system.
TABLE 1TDD uplink-downlink configurationsUplink-Downlink-to-uplinkDownlinkswitch-pointsubframe numberconfigurationsperiodicity012345678905msDSUUUDSUUU15msDSUUDDSUUD25msDSUDDDSUDD310msDSUUUDDDDD410msDSUUDDDDDD510msDSUDDDDDDD65msDSUUUDSUUD
TABLE 2Special subframe configurations (length of DwPTS/GP/UpPTS)Normal CP, downlinkExtended CP, downlinkUpPTSUpPTSSpecialNormalExtendedNormalExtendedsubframeCP,CP,CP,CP,configurationsDwPTSuplinkuplinkDwPTSuplinkuplink0 6592 · Ts2192 · Ts2560 · Ts 7680 · Ts2192 · Ts2560 · Ts119760 · Ts20480 · Ts221952 · Ts23040 · Ts324144 · Ts25600 · Ts426336 · Ts 7680 · Ts4384 · Ts5120 · Ts5 6592 · Ts4384 · Ts5120 · Ts20480 · Ts619760 · Ts23040 · Ts721952 · Ts———824144 · Ts———
Both the TDD mode and the FDD mode have its own advantages and disadvantages. For example, the uplink/downlink configurations of the TDD system as shown in Table 1 are configurable and may better support the asymmetric services and improve usage efficiency of frequency spectrums, while at a pair of FDD frequency spectrums, uplink frequency spectrum resources is wasted when there are many downlink services. However, the FDD uplink and downlink resources are always available due to the paired FDD frequency spectrums, so that a terminal may timely return uplink control signaling, such as an Acknowledge/Non-Acknowledge (ACK/NACK) message of a Hybrid Automatic Retransmission Request (HARQ) and Channel state information (CSI), and therefore feedback delay of an air interface can be reduced and the scheduling efficiency can be improved.
In addition, an uplink channel and a downlink channel of the TDD system use the same carrier and the uplink and downlink radio channels are almost the same. As such, after a base station receives an uplink signal and obtains uplink channel quality measurement or estimation, a state of the downlink channel is obtained, such as the speed of fading, adjacent areas interference, etc. This feature is called channel reciprocity. Since an uplink link and a downlink link in the FDD use different frequency bands, an uplink radio channel and a downlink radio channel are different. In order to obtain downlink channel information, a base station may send a downlink reference signal (RS). A terminal estimates downlink channel quality or channel fading according to the reference signal and returns estimated information. The base station performs operations like downlink scheduling, resource allocation, pre-coding based on the returned downlink channel information. Because the TDD system achieves the channel reciprocity, a multi-antenna system can be better used.
As can be seen that in a future radio communication system, the advantages of the TDD system may be integrated into the FDD system, so that the features of the TDD including the channel reciprocity and supporting the asymmetric services can be used by the FDD system. In this way, the usage efficient of the frequency spectrums and network performance can be greatly improved. In the FDD system integrating the advantages of the TDD, an uplink (or downlink) work carrier of the FDD system transmits uplink data and downlink data in a Time Division Multiplexing (TDM) manner. Various embodiments of the present disclosure describe transmission of a sounding reference signal in such system.